Tens of thousands of dead fish washed ashore on a South Carolina beach on Tuesday, for at least the second time in a week.

Approximately 30,000 to 40,000 menhaden fish washed ashore along a mile-and-a-half-long stretch of beach from Georgetown County, S.C., to Pawleys Island, said Pawleys Island Police Chief Michael Fanning, according to NBC News. It seemed likely that thousands more of the 6-to 8-inch-long fish would wash ashore later.

This is the second time in one week such an incident has occurred in the area, notes NBC News. Late last week, hundreds of thousands of the small, oily fish washed up near Masonboro Island, N.C.

A local sergent offered this explanation for the bizzare beach full of fish:

“It’s an occurrence called hypoxia,” Sergeant Steve Pop told WMBF.

That is dissolved oxygen levels in the water that drop to a level that is not sustainable for the fish… We’ve got some deep holes off of north inlet that holds these fish.

This time of year the fish congregate in these holes which is depleting the oxygen source.”